Packet data latency is one of the performance metrics that vendors, operators and also end-users (via speed test applications) regularly measure. Latency measurements are done in all phases of a radio access network system lifetime, when verifying a new software release or system component, when deploying a system and when the system is in commercial operation.
One performance metric that guided the design of Long Term Evolution, LTE, was better latency than previous generations of Radio Access Technologies, RATs, defined by the Third Generation Partnership Project, 3GPP. LTE is also now recognized by the end-users to be a system that provides faster access to Internet and lower data latencies than previous generations of mobile radio technologies.
Packet data latency is important not only for the perceived responsiveness of the system; it is also a parameter that indirectly influences the throughput of the system. Hyper-Text Transport Protocol/Transport Control Protocol, HTTP/TCP, is the dominant application and transport layer protocol suite used on the Internet today. According to HTTP Archive, http://httparchive.org/trends.php, the typical size of HTTP-based transactions over the Internet range from a few 10s of Kbytes up to 1 Mbyte. In this size range, the TCP slow start period is a significant part of the total transport period of the packet stream. During TCP slow start, the “congestion window” used by TCP for defining the amount of traffic that can be outstanding, i.e., transmitted but not acknowledged, and packet latency limits how quickly the congestion window can be optimized. Hence, improved latency improves the average throughput for these types of TCP-based data transactions.
Radio resource efficiency in general is positively impacted by latency reductions. Lower packet data latency could increase the number of transmissions possible within a certain delay bound; hence higher Block Error Rate (BLER) targets could be used for the data transmissions freeing up radio resources potentially improving the capacity of the system.
Here, a “scheduling interval” is the smallest unit of time allocated when scheduling resources. In LTE, scheduling intervals are referred to as Transmission Time Intervals (TTI). One area to address when it comes to packet latency reductions is the reduction of transport time for data and control signaling, by addressing the length of a TTI. In LTE release 8, a TTI corresponds to one subframe (SF) of length 1 millisecond. One such 1 ms TTI is constructed by using 14 OFDM (Orthogonal Frequency Division Multiplexing) or SC-FDMA (Single Carrier Frequency Division Multiple Access) symbols in the case of normal cyclic prefix (CP) and 12 OFDM or SC-FDMA symbols in the case of extended CP. For LTE release 13, 3GPP is studying the use of transmissions in TTIs that are much shorter than the LTE release 8 TTI.
For this disclosure, it is assumed that the TTIs may be shortened compared to the release 8 TTI, by introducing a sub-subframe (SSF) concept, also denoted short TTI (sTTI). These shorter TTIs or sTTIs (also known as SSFs) can be decided to have any duration in time and comprise resources on a number of OFDM or SC-FDMA symbols within a 1 ms SF. As one example, the duration of the SSF may be 0.5 ms, i.e. seven OFDM or SC-FDMA symbols for the case of normal CP.
Uplink Scheduling Grants
The existing physical layer downlink control channels, Physical Downlink Control Channel (PDCCH) and enhanced PDCCH (ePDCCH), are used to carry Downlink Control Information (DCI) such as scheduling decisions and power control commands. Both PDCCH and ePDCCH are transmitted once every subframe (SF) of 1 ms. Throughout this disclosure, short PDCCH (sPDCCH) is used to denote downlink physical control channels transmitted once every SSF. Similarly, short Physical Downlink Shared Channel (sPDSCH) and short Physical Uplink Shared Channel (sPUSCH) are used to denote the downlink and uplink physical shared channels transmitted once every SSF, respectively.
There are currently a number of different Downlink Control Information (DCI) formats for uplink and downlink resource assignments, as specified in 3GPP TS 36.212 (Rel 10) V 12.6.0, section 5.3.3.1. Uplink scheduling grants use either DCI format 0 or DCI format 4. The latter DCI format 4 is added in Release 10 for supporting uplink spatial multiplexing.
In general, the DCI for an uplink (UL) scheduling grant contains:                Resource allocation information                    Carrier indicator            Resource allocation type            Resource block allocation                        RS and data related information                    Modulation and coding scheme (MCS)            New data indicator            Cyclic shift of the uplink demodulation reference signals (DMRS)            Precoding information            Transmit power control                        Other information                    Sounding Reference Signal (SRS) request            Channel State Information (CSI) request            UL index (for Time Division Duplex (TDD))            DCI format 0/1A indication (only in DCI format 0 and 1A)            Padding            Cyclic Redundancy Check (CRC) scrambled with Radio Network Temporary Identifier (RNTI) of the terminalDynamic Switching Between SSF Lengths                        
As mentioned, one way to reduce latency is to reduce the TTI. Instead of assigning resources for a time duration of 1 ms, i.e. for a subframe, resources may be assigned for a shorter duration than one subframe, i.e. for a SSF. The shorter duration or SSF may e.g. be defined in number of OFDM or SC-FDMA symbols. This implies a need for UE (User Equipment) specific control signaling that enables an indication of such shorter scheduling assignments.
Furthermore, there is also a need to be able to dynamically switch between different TTI or SSF durations, such as between the legacy 1 ms TTI and shorter TTIs, as well as between different shorter TTIs. This is needed in order to optimize the spectral efficiency, since shorter TTIs may incur higher overhead and/or worse demodulation performance.
Potential Problem with Existing Approaches
The existing way of operation, e.g. frame structure and control signaling, are designed for data allocations in a fixed length subframe of 1 ms, which may vary only in allocated bandwidth. Specifically, the current DCIs define resource allocations within the entire subframe.